Portable electronic devices such as laptop computers, cell phones, pagers, personal digital assistants, and the like are becoming more common in today's society as the capabilities and uses of such devices continues to expand. Many portable electronic devices are powered by a rechargeable battery, e.g., lithium, nickel-cadmium, or nickel-metal hydride type batteries, to facilitate the portable nature of such devices. Such portable electronic devices may also be powered by a DC power source when the situation permits, e.g., an AC/DC adapter plugged into a conventional AC outlet. Such a DC power source may also provide power to recharge the rechargeable battery in a battery charging mode.
In such a battery charging mode, various portable electronic devices may also have a DC to DC converter to accept unregulated power from the DC power source and to provide regulated DC power to recharge the rechargeable battery. The DC to DC converter may be controlled by a DC to DC controller. The DC to DC controller may accept a variety of input signals representative of various supply and charging conditions. For instance, one input may be representative of the DC power source supply current, another may be representative of an output charging current, and yet another representative of an output charging voltage. The DC to DC controller may also have a variety of control path or loops associated with each input signal, e.g., an input supply current control path, an output charging current control path, and an output charging voltage control path. The DC to DC controller then provides a control signal to the DC to DC converter based on at least one of these input signals to control the output charging power level to the rechargeable battery.
Some rechargeable batteries have an internal switch that when open electrically isolates the battery cells from other components, e.g., the DC to DC converter if the battery is being recharged. Such an internal switch may open in a variety of circumstances. For example, such a switch may open for self-calibration reasons so the battery can monitor its voltage levels on its battery cells without a flowing current. Such a switch may also open for protection reasons, e.g., when instantaneous power delivered to the battery exceeds the maximum allowed power for the battery.
The opening and closing of such a switch can cause in-rush current problems. For instance, when such a switch is open during a battery charging mode, the DC to DC controller senses that the charging current has decreased to zero. In response to this sensed condition, the DC to DC controller may increase the DC to DC output voltage level until it reaches some predetermined maximum level. Then, once the battery's internal switch is closed again, a large in-rush current may be created due to the potential difference between the output voltage of the DC to DC converter and the voltage of the battery. Such a large in-rush current may lead to failure or degradation of the rechargeable battery and associated electronics. The amplitude of the in-rush current depends primarily on the magnitude of the voltage difference. The duration of the in-rush current depends on a number of factors including the speed of the current control path or loop of the DC to DC controller and the capacitance value of an output capacitor to name a couple.
Accordingly, there is a need for a DC to DC controller and method that overcomes the above deficiencies in the prior art and is capable of controlling in-rush current.